The present invention relates to electrochemical probes, and more particularly to electrochemical sensors for measuring certain characteristics of liquids, particularly aqueous liquids, and components of such sensors.
In many situations, it is desirable to monitor a variety of water quality parameters, often at frequent intervals or even continuously. Water quality parameters include, for example, temperature, pH, free chlorine, total alkalinity, hardness, total dissolved solids, and oxidation reduction potential (ORP). Many of these parameters can be measured electrolytically. For ease of explanation, much of the following discussion will be with reference to monitoring water quality in a swimming pool, although it should be borne in mind that the discussion is likewise applicable to monitoring water quality in other settings, such as a spa, Jacuzzi™, hot tub, fountain, aquarium, sprinkler system to spray produce, water tank, or cooling tower.
Swimming pool water must be monitored vigilantly to ensure that the water is clean and safe for use. Conventionally, this is a manual process carried out by the owner or other caretaker of the pool. The process involves going to the pool with vials and chemicals, scooping water into vials, shaking the vials, and comparing the color of the resulting solutions to those on charts to determine the chemicals needed to restore proper pool chemistry. After testing, it is necessary to obtain the chemicals, measure them out, and add them to the pool water. For example, the chlorine disinfectant used to sanitize the pool may have been depleted by a heavy bather load, and more chlorine needed to destroy algae, waterborne germs, and oxidize organic debris introduced into the pool water by swimmers. It is a cumbersome process, and if pool water quality is not maintained properly, the swimmer can contract waterborne illnesses such as diarrhea, swimmer's ear, and skin infections.
The maintenance of swimming pool water is multifaceted in the number of factors that must be controlled. Scheme A shows water quality parameters recommended for swimming pool water.
Scheme AWater Quality ParameterIdeal LevelpH7.2 to 7.8Free Chlorine1.0 to 3.0 ppmTotal Alkalinity (buffering capacity)80 to 120 ppmSalt2,700 to 3,400 ppmStabilizer60 to 80 ppmHardness200 to 400 ppmTotal Dissolved SolidsLess than 6,000 ppmOxidation-Reduction Potential650 mV
Two critical factors in maintaining water balance are pH and free chlorine level (FCL). pH is a measurement of the concentration of hydrogen ions in water. It is measured using a logarithmic scale from 0 to 14, with pH 7 being neutral. For pool water to be in balance, the pH must be maintained at a level between 7.2 and 7.8. At pH below 7.2, the water is considered to be corrosive and can etch plaster and metal in equipment such as heat exchangers. Maintaining the pH higher than 7.8 will increase the tendency to form scale or cloudy water due to precipitation of calcium dissolved in the water. Higher pH will also render chlorine sanitizers ineffective, as discussed further below.
Addition of chlorine sanitizers such as aqueous sodium hypochlorite solution (bleach) or solid calcium hypochlorite to water generates a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl−) known as “free chlorine.” The pool industry typically recommends that a free chlorine concentration of between 1.0 and 3.0 ppm be maintained in the swimming pool to provide for effective sanitation. Hypochlorous acid is a more effective disinfectant and oxidant than the hypochlorite ion, and their relative proportions fluctuate with the pH of the water in the pool (low pH is more acidic and high pH is more basic). At high pH, free chlorine will be mostly in the form of hypochlorite and so it will be less effective as a sanitizer. Thus, measuring free chlorine alone does not assure efficacy. Both the pH and free chlorine levels of swimming pool water must be monitored to ensure that an adequate water quality level is maintained.
Chlorine and bromine are both members of the same chemical family known as halogens. While not as popular as chlorine, bromine has gained wide acceptance as a sanitizer, especially in hot tubs where the hot turbulent water tends to increase the amount of wastes in the water. Bromine tablets, sticks or caplets are usually applied through some type of feeder device either in-line or, in some cases, as a floater-type feeder. The two-product system relies upon the addition of small amounts of an inert sodium bromide salt, which by itself does little. The water is then treated with an oxidizer especially suited for this purpose, or with chlorine. The oxidizer or chlorine acts to convert sodium bromide into “free bromine”, a mixture of hypobromous acid (HOBr) and hypobromite ion (OBr−). However, unlike chlorine, the amount of hypobromous acid present is less dependent on pH. Additionally, the bromamines formed when HOBr reacts with waste in the water do not cause eye and skin irritation or foul odors.
Alternatively, there is another way of testing the water called oxidation-reduction potential (ORP). ORP is a measure, in millivolts, of the tendency of a chemical substance to oxidize or reduce another chemical substance. A positive voltage indicates an oxidizing solution and a negative voltage indicates a reducing solution. ORP measurements are valid over a wide pH range, and provide an index of water quality based on activity of a sanitizer rather than just its quantity. The lower oxidation potential of bromine compared to chlorine means that ORP will not be as sensitive to the concentration of bromine as it will to chlorine. In 1988 the National Swimming Pool Institute adopted a standard of ORP value of greater than or equal to 650 mV for public spas. An ORP greater than or equal to 650 millivolts is adequate to kill viral and bacterial pathogens within seconds.
Another swimming pool water parameter that is important to determine is the amount of total dissolved solids (TDS). TDS is the sum of all materials dissolved in the water, and normally runs in the range of 250 ppm and higher. TDS can be salts like sodium chloride and calcium chloride, metals like iron, copper, and manganese, and dissolved organic compounds. The guideline for the maximum amount of total dissolved solids allowed in pool/spa water is <6000 ppm and in at least some environments <1500 ppm. At elevated levels, TDS can lead to cloudy or hazy water, difficulty in maintaining water balance, reduction in sanitizer activity, and foaming. It can also inhibit the sanitizer efficiency to the point that algae plumes form even though tests indicate an acceptable free chlorine level. When this problem is identified, the only way to reduce TDS is to drain a portion of the water and replace it with fresh water.
It is desirable to have sensors that can monitor water quality parameters automatically and frequently, even continuously. Sensors for such measurements often operate on a potentiometric electrochemical principle that incorporates a reference electrode and a sensing electrode. Conventional reference electrodes for use in such potentiometric electrochemical measurements typically incorporate an internal reference fill solution in contact with an electrode in contact with a test solution through a porous junction, which allows a slow leak of the internal reference fill solution to provide the necessary electrolytic contact with the liquid being tested. A metal or electrochemical electrode in contact with the test solution completes the circuit and an electrical potential on the reference electrode remains relatively constant while the sensing electrode responds to chemical changes in the test solution.
Conventional sensors of this type suffer from several drawbacks when applied to certain measurement environments. One problem is that chemicals employed to sanitize the water can interfere with the measurements. Conventional electrodes of this type suffer from several drawbacks when applied to certain measurement environments, such as long-term unattended monitoring of pool or spa water. For example, because leakage of the internal reference fill solution through the porous junction into the tested environment is necessary to provide electrolytic conductivity between the internal reference fill solution and the tested environment, the useful life of the reference electrode is limited. Moreover, a high rate of leakage is desirable to produce a low electrical impedance of the reference electrode. Moreover, in a pipe mounted system, the flow of test solution flowing over the electrode exacerbates the high leakage rate. Thus, while low electrical impedance is desirable for accurate measurements because it reduces noise, the high leakage rate employed in such conventional reference electrodes to produce the desired low electrical impedance severely limits the life of the electrode and the electrode must be frequently refilled with fresh internal solution or replaced.
Such conventional reference electrodes suffer from other disadvantages as well. For example, they tend to be fragile, typically being encased in glass. Moreover, they often are limited in operational orientation. In other words, because the reference fill solution of the electrode is a liquid, it readily flows as a result of gravity. Thus, the relative orientation of the electrode with respect to the reference fill solution and the reference fill solution with respect to the porous junction depends on the spatial orientation of the electrode and so the electrode assembly in the test solution must be oriented vertically so that the reference fill solution is properly oriented in the electrode. Indeed, silver/silver chloride reference electrodes suspended in glass-encased fill solutions have been employed in combination with antimony electrodes in some pH sensors, with all the attendant disadvantages of glass membranes and fill solutions noted above.
What is needed are sensors that are precise and reliable and can accurately monitor various water quality parameters such as pH, free chlorine level, ORP, and TDS over an extended period of time. In particular, it would be desirable to have sensors that are individually calibrated and have their own memory device. Further, it would be desirable to interface such sensors with control equipment so that appropriate water quality adjustments can be made to ensure the water is healthful. The plurality of sensors of the present invention is robust and provide for long periods of unattended operation.